Decorative laminating papers and decorative foils are part of a decorative, thermosetting coating material, used with preference for finishing furniture surfaces, for laminate flooring and in interior finishing. Laminates is the term used to denote materials in which, for example, several impregnated, stacked layers of paper, or paper and hardboard or fibreboard, are pressed together. The use of special synthetic resins achieves extraordinarily high resistance of the laminates to marring, impact, chemicals and heat.
The use of decorative laminating papers (which is always also intended to mean decorative foils below) makes it possible to produce decorative surfaces, where the decorative laminating paper serves not only as facing paper, e.g. to hide unattractive wood material surfaces, but also as a carrier for the synthetic resin.
The demands imposed on decorative laminating paper include, among others, opacity (hiding power), light-fastness (greying resistance), colour-fastness, wet strength, suitability for impregnation and printability.
The economic efficiency of the manufacturing process for decorative laminating papers is determined by, among other things, the opacity of the pigment in the paper. In principle, a pigment based on titanium dioxide is eminently suitable for achieving the necessary opacity of the decorative laminating paper. During paper manufacture, a titanium dioxide pigment, or a titanium dioxide pigment suspension, is usually mixed with a pulp suspension. In addition to pigment and pulp as the feedstock, use is generally also made of auxiliaries, such as wet-strength agents, and further additives where appropriate. The interactions of the individual components (pulp, pigment, auxiliaries and additives, water) with each other contribute to formation of the paper and determine the retention of the pigment. Retention is the capacity for retaining all inorganic substances in the paper during production. The surface charge of the pigment in relation to the pulp fibre plays an important role in this context.
It is known that an improvement in opacity can be achieved by special treatment of the surface of the titanium dioxide pigment.
EP 0 713 904 B1 describes a surface treatment in which a first layer of aluminium oxide phosphate is applied at an acidic pH value of 4 to 6, and a second layer of aluminium oxide is precipitated in a pH range from 3 to 10, preferably at roughly pH 7. An improvement in retention is achieved by a third layer consisting of magnesium oxide, the result being that the pigment produced is characterised by consecutive layers of aluminium oxide phosphate, aluminium oxide and magnesium oxide.
US 2004/0025749 A discloses a method for the surface treatment of a titanium dioxide pigment, in which a component containing phosphorus, titanium and aluminium is applied first by precipitation, followed by a magnesium component at a pH value of 8 to 10. These pigments are said to display improved light-fastness and high opacity.
US 2005/0011408 A describes a method for the surface treatment of a titanium dioxide pigment, in which an aluminium component and a phosphorus component are added to the TiO2 surface treatment suspension at a pH value of at least 10 and subsequently precipitated at a pH value below 9. The method is said to lead to improved retention and unchanged opacity.
U.S. Pat. No. 6,200,375 discloses a weather-resistant titanium dioxide pigment for outdoor coatings, where the surface of the particles displays consecutive layers of zirconium hydroxide, titanium hydroxide, phosphate/silicon oxide and hydrous aluminium oxide.